1. Field of the Invention
The present invention relates to magnetoresistive (MR) sensor biasing. More particularly, the present invention relates to magnetoresistive sensor biasing and read-out methods and circuits.
2. Description of the Related Art
FIG. 1 shows a conventional biasing arrangement 10 for a dual MR head used for detecting variations in a magnetic field on a disk (not shown), for example. A bias current I.sub.B flowing in each of two insulated MR sensors elements 11 and 12, which are placed in close proximity to each other, provides equal and opposite magnetic bias M.sub.1, M.sub.2, respectively, for the two elements. Current source 13 supplies I.sub.B to MR sensor element 11 through a conductor at 15, while current source 14 supplies I.sub.B to MR sensor element 12 through a conductor at 16. MR sensor elements 11 and 12 are connected together by a conductor at 17 forming a common node that is connected to ground. The two bias currents are combined at node 17 so that 2I.sub.B flows to ground.
Variations in the disk magnetic field detected by sensor elements 11 and 12 are conventionally read out as indicated in FIG. 1. An amplifier 18 is used for detecting potential differences caused by changes in sensor element resistance resulting from variations in the disk magnetic field. The detected potential differences are output by amplifier 18 as V.sub.OUT.
This conventional dual MR-sensor structure provides immunity against thermal asperities while also providing, in theory, a large Common-Mode Rejection Ratio (CMRR), making this sensor structure insensitive to (common-mode) interference caused, for example, by interference capacitively injected into the head. However, the CMRR of the head is limited by any relative imbalance of the resistances R.sub.1 and R.sub.2 of MR sensor elements 11 and 12, respectively.
FIG. 2 shows a schematic block diagram used for calculating a CMRR of a dual MR sensor head. In FIG. 2, a common mode voltage V.sub.cm is applied through common mode impedances Z.sub.cm to sensor element resistances R.sub.1 and R.sub.2. From FIG. 2, the interference sensitivity for a single-ended MR-sensor configuration (i.e., R.sub.2 =0) using a single-ended input amplifier 18 is ##EQU1##
For a differential MR-sensor configuration, that is, the dual sensor configuration (R.sub.1, R.sub.2) shown in FIG. 1, the interference sensitivity is ##EQU2## Hence, the common-mode rejection ratio of the single-ended configuration over the differential configuration is ##EQU3## and depends on the relative resistance match R.sub.2 /R.sub.1 of the two sensor elements 11 and 12. For R.sub.1 =R.sub.0 (1.+-..epsilon.) and R.sub.2 =R.sub.0 (1.+-..epsilon.) EQU CMRR=(1+.epsilon.)/2.epsilon..congruent.1/2.epsilon. (5)
For .epsilon.=5%, the CMRR is therefore only 10 (or 20 dB). CMRR values for conventional differential amplifiers, such as those used for amplifier 18, are typically on the order of 1000-2000, or 60-66 dB. Consequently, the overall CMRR of a conventional dual MR-sensor head with an electronic read-out as in FIG. 1 is primarily controlled by the resistance matching of the MR-sensors.
Further, conventional biasing arrangements provide that the disk and the common node 17 are at the same potential. Thus, any contact of a sensor element with the grounded disk results in the bias current I.sub.B flowing into the short-circuit with the disk causing a high spot-temperature and erosion of the sensor element.